One major area of psychology is the development of personality theories. Personality is the totality of characteristics in an individual, uniquely influencing one's cognition, emotion, motivation, and behavior. Personality theory is based on the idea that people are similar in many ways, yet different in others. There are several basic schools of personality theory: trait, type, psychoanalytic, behaviorist, social cognitive, humanistic, and evolutionary.
Trait theories hold that personality traits, enduring patterns of behavior, act along a bipolar continuum. People are either friendly or unfriendly. They are either extroverts or introverts. Type theories, which originated with Carl Jung, see personality in more than two shade. The Meyers-Briggs Type Indicator is a practical test that attempts to measure personality.
Freud is famous for his psychoanalytic personality theory, dividing the human personality in the id, the ego, and the superego. Behaviorists, on the other hand, explain personality as resulting from the effects of external stimuli. Notable behaviorists include Ivan Pavlov and B.F. Skinner. Albert Bandura developed social cognitive personality theory, suggesting that memory and emotion worked in conjunction with the environment. Abraham Maslow and Carl Rogers both proposed humanistic personality theories, while evolutionary personality theory starts with the work of Charles Darwin.